birthmark, pigmented maldevelopment of the skin that varies in size, either present at birth or developing later. Birthmarks may appear as moles (melanocytic nevi) that vary in color from light brown to blue, and are either flat or raised above the surface of the skin. They are usually benign, but do rarely develop into malignant melanoma, a form of skin cancer. The so-called port-wine stains and strawberry marks involve vascular tissue. The flat port-wine stains can be made lighter with the use of laser therapy. The strawberry marks generally disappear a few years after birth.
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